The 3rd-Generation Partnership Project (3GPP) specifications refer to end-user wireless communication devices as “User Equipment” (UEs). UEs are also known as mobile terminals, wireless terminals, mobile stations, etc., and are configured to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed, e.g., between two UEs, between a UE and a regular telephone, and/or between a UE and a server, via a Radio Access Network (RAN) and possibly one or more core networks that together make up the cellular communications network.
Various examples of and/or alternative names for UEs include mobile telephones, cellular telephones, laptops, or table computers with wireless capability, to name a few examples. UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as with another UE or with a server. The concept of “user equipment” also includes devices with communications capability of a machine-type character, such as wireless-enabled sensors, measurement devices, etc., where the device is not necessarily interacting with a human user at all.
A cellular communications network covers a geographical area that is divided into cell areas, where each cell area is served by a base station, e.g., a Radio Base Station (RBS). An RBS may sometimes be referred to as, e.g., “base station”, “eNodeB”, “NodeB”, “B node”, or “BTS” (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes, such as macro eNodeBs, home eNodeBs or pico base stations, where the classification is based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site.
Carrier Aggregation
Carrier aggregation (CA) is one of the ways recently developed by the 3GPP for increasing the per user throughput for users that have good channel conditions and that have the capability of receiving and transmitting at higher data rates than might be supported by a single carrier. With carrier aggregation, a user can be configured to operate in two or three (or more) simultaneous bands in the downlink and/or in the uplink.
FIG. 2 illustrates the example of an eNB 120 that is capable of running four different cells at the same time. Each arrow in a given direction indicates a “carrier,” which is a set of associated channels that will support a link in that direction, thus providing a “cell” for the UE. The carriers that are aggregated in carrier aggregation (CA) are referred to as “component carriers” (CCs). Each group of associated arrows, or CCs, indicates one of several frequency bands, which, for a given direction, are typically not immediately adjacent to one another in frequency. Each frequency band may support one or more carriers; in the examples illustrated in FIGS. 2A-2D, there are two frequency bands, each of which supports two uplink carriers and two downlink carriers.
In the illustrated examples, the shaded arrows indicate the carriers that are supported by the illustrated UE 110. In any given scenario, depending on the capabilities of the eNB and the UE, cells may be operated in different bands or in the same band. Note that in Release 8 of the LTE specifications, i.e., prior to the introduction of CA, only one cell is used for communication between eNB and UE. Operation according to Release 8 behavior is shown in FIG. 2A, while several different configurations of CA are shown in FIGS. 2B, 2C, and 2D.
More particularly, FIG. 2B illustrates downlink (DL) CA, with two downlink carriers, in two different frequency bands, combined with a single uplink carrier. This can be referred to as “2DL CA” or “CA with 2 DL CCs.” This is the initial version of downlink carrier aggregation. In this case, the UE is configured to receive in two downlink bands simultaneously, while using uplink in only one of the bands. The uplink allocation in this case is arbitrary, meaning that either of the bands can be used for uplink transmission.
FIG. 2C illustrates downlink CA on three carriers, across two frequency bands, again combined with a single uplink carrier. This might be referred to as “3DL CA” or “CA with 3DL CC and 1 UL CC.” Three downlink bands can be allocated to any given UE, thus realizing 3DL carrier aggregation, as shown in the figure. As with the 2DL case, the uplink can be allocated to any of the bands.
FIG. 2D illustrates two-carrier CA in both the uplink and downlink directions, this time confined to a single band. Thus, FIG. 2D shows the case where uplink (UL) carrier aggregation is also enabled for the terminal. This might be referred to as “2UL CA” or “CA with 2 UL CCs and 2 DL CCs.” More generally, 3 DL CCs may be activated along with the 2 UL CCs.
In carrier aggregation terms, the cell where uplink is allocated for a given UE is the PCell (primary cell), while the other aggregated cell is SCell (secondary cell). PCell and SCell combinations are UE-specific. In the case of uplink carrier aggregation, PCell and SCell definitions are still UE-specific.
Depending on the carrier frequencies, or depending on the physical eNB deployment, the deployment of different CA-enabled systems can be very different. FIG. 3 illustrates two examples of CA deployment. The left hand side of FIG. 3 shows a deployment in which F1 and F2 cells are co-located and overlaid, where F1 and F2 represent carriers at different frequencies. F2 has smaller coverage, due to larger path loss. In this deployment, only F1 provides coverage over the entire deployment area; F2 is used to improve throughput. Mobility is performed based on F1 coverage. A likely scenario is that F1 and F2 are of different bands, e.g., F1={800 MHz, 2 GHz} and F2={3.5 GHz}, etc. Thus, it is expected that aggregation is possible between directly overlaid F1 and F2 cells.
The deployment illustrated on the right hand side of FIG. 3 shows a different kind of deployment. In this case, F1 provides macro coverage and Remote Radio Heads (RRHs) operate the F2 carriers and are used to improve throughput at hot spots. Mobility in this deployment is performed based on F1 coverage. Again, a likely scenario is that F1 and F2 are of different bands, e.g., F1={800 MHz, 2 GHz} and F2={3.5 GHz}, etc. In a deployment of this type, it is expected that the F2 cells provided by the RRHs can be aggregated with the underlying F1 macro cells.
Dual Connectivity
A dual connectivity framework is currently being considered by 3GPP for Release 12 of the standards for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), which is widely referred to as Long Term Evolution (LTE). Dual connectivity refers to a mode of operation in which a given UE, while in RRC_CONNECTED mode, consumes radio resources provided by at least two different network points connected to one another with a non-ideal backhaul. In the LTE standards, these two network points may be referred to as a “Master eNB” (MeNB) and a “Secondary eNB” (SeNB). Given the above discussions, it should be appreciated that dual connectivity can be viewed as a special case of carrier aggregation, where the aggregated carriers, or cells, are provided by network nodes that are physically separated from one another and that are not connected to one another through a fast, quality, connection.
A UE in dual connectivity maintains simultaneous connections to anchor and booster nodes, where the anchor node is also called the master eNB (MeNB) and the booster nodes are also called secondary eNBs (SeNB). As its name implies, the MeNB controls the connection of the UE and handovers to and from SeNBs. No SeNB standalone handover is defined for Release 12. Signaling in MeNB is needed even in SeNB change. The MeNB terminates the control plane connection towards the UE and can thus be the controlling node of the UE. However, the MeNB can also configure the UE based on input from the SeNB and hence the SeNB can indirectly also control the UE.
The UE reads system information transmitted by the anchor node. In addition to the anchor node, the UE may be connected to one or several booster nodes, for added user plane support. The MeNB and SeNB are connected to one another via the Xn interface, which is currently selected to be the same as the X2 interface between the two eNBs.
More specifically, dual connectivity (DC) is a mode of operation of a UE in RRC_CONNECTED state, where the UE is configured with a Master Cell Group (MCG) and a Secondary Cell Group (SCG). A Cell Group (CG) is a group of serving cells associated with either the MeNB or the SeNB. The MCG and SCG are defined as follows:                The Master Cell Group (MCG) is a group of serving cells associated with the MeNB, comprising a primary cell (PCell) and optionally one or more secondary cells (SCells).        A Secondary Cell Group (SCG) is a group of serving cells associated with the SeNB comprising a Primary Scell (pSCell) and optionally one or more SCells.        
The Master eNB is the eNB that terminates at least the S1-MME connection, i.e., the connection between the eNB and the Mobility Management Entity (MME) for the UE. A Secondary eNB is an eNB that is providing additional radio resources for the UE but that is not the Master eNB.
FIG. 1 illustrates an example of a dual connectivity setup, in which several dual connectivity scenarios are illustrated among UEs 110 and base stations (eNBs) 120. In this example, only one SeNB (at most) is connected to any of the illustrated UEs. However, more than one SeNB can serve a UE in general. Moreover, only one cell each from both MeNB and SeNB are shown to be serving the UE, however more than one cells can serve the UE in practice from both MeNB and SeNB. From the figure, it should also be clear that dual connectivity is a UE-specific feature and that a given network node (or a serving cell) can support a dual-connected UE and a legacy UE at the same time. In other words, MeNB and SeNB are roles played, or functions provided, by eNBs 120 for a given situation. Thus, while the eNBs 120 in FIG. 1 are labeled “MeNB” and “SeNB,” this indicates only that they are playing this role for at least one UE 110. Indeed, a given eNB 120 may be an MeNB for one UE 110 while being an SeNB for another.
Thus, the master/anchor and secondary/booster roles are defined from a UE's point of view, which means that a node (or cell) that acts as an anchor to one UE may act as booster to another UE. Likewise, although a given UE in a DC scenario reads system information from the anchor node (or cell), a node (or cell) acting as a booster to one UE may or may not distribute system information to another UE.
In summary, then, the MeNB provides system information, terminates the control plane, and can terminate the user plane. An SeNB, on the other hand, terminates only the user plane.
Dual connectivity allows a UE to be connected to two nodes to receive data from both nodes to increase its data rate. This user plane aggregation achieves benefits that are similar to those provided by carrier aggregation, which is described below, while using network nodes that are not connected by low-latency backhaul/network connection. Due to this lack of low-latency backhaul, the scheduling and HARQ-ACK feedback from the UE to each of the nodes will need to be performed separately. That is, it is expected that the UE shall have two UL transmitters to transmit UL control and data to the connected nodes.